Mechanical temporary fasteners are used in a number of industries to temporarily hold workpieces or sheets of material together prior to fastening. Particularly, mechanical temporary fasteners are used extensively in the aircraft manufacturing industry to precisely join assorted structural members for final assembly.
One such typical temporary fastener includes elongated, slidable, needle-like structures or fingers with enlarged heads that are inserted into aligned apertures located in the sheet layers to be fastened. Clamping pressure is applied to the layers by turning a threaded sleeve that retracts the splayed fingers over a spreader located between the fingers. The sheets are secured by forced separation of the enlarged heads thereby causing a wedged seal within the aperture. Such fasteners, in theory, develop a proportionate clamping force in response to the torque applied to the screw threads. In actual practice, that relationship is often catastrophically altered by other variables such as friction, foreign materials, installation speed, etc. In practice, it is difficult or nearly impossible to utilize a mechanical fastener to obtain a consistent or desired clamping pressure or load to the structures.
Because clamping pressure is relative to the degree of torque applied to the fastener, too much torque may cause an increased clamping pressure that may damage the layers while too little torque may provide inadequate pressure resulting in poor fastening. For example, the “back-side” clamping feature (the protuberances on the fingers) has an extremely small foot print area since it must fit through aligned apertures. When clamping force is excessive it may cause unacceptable damage to the “back-side” component material where contact is made by the clamping feature. A lack of clamping pressure control is inherent in the design of such mechanical temporary fasteners and may result in unacceptable damage to structural materials.
Further, a popular version of the prior art temporary fastener described above has its use limited to clamping work pieces that are dry. When the work pieces are “wet” (when the work pieces have a viscous fluid such as an adhesive or a dissimilar metals barrier medium therebetween) the clamp-up is likely to disappear due to a modest migration of that fluid via hydraulic reaction to clamping force. A companion to that fastener device is designed with a mechanical spring to compensate for that migration. The enhanced merits of both designs are inherent features of the present invention. Therefore, there is a need in the art to provide a temporary fastener that enables a user to more precisely control the amount of clamping force developed.
Another type of typical temporary fastener achieves its clamping action by spring biasing the fingers having enlarged heads toward the clamp body over a spreader bar where the fingers are normally withdrawn into the clamp body. The clamp is operable by overcoming the spring bias and extending the needles over and beyond a spreader bar, which allows the flexible head ends to be moved toward each other for insertion into aligned holes. The clamp body is subsequently brought into proximity to the workpiece surface, either by the operator or by reaction to the spring pulling the needle members into the clamp body during needle member withdrawal through the clamp body. The force applied to overcome the spring bias is removed and the spring urges the needles within the clamp body and over the spreader bar to force the needle ends to move apart to engage the inner panel and inner surface, thus securing the workpieces between the clamp body and the enlarged ends of the needles. Needless to say, the maximum clamping force that the clamp exerts is in some cases limited by the spring force, that is the product of the spring constant and the spring displacement. This spring force may not be adequate to clamp the two sheets together.
Regardless of which prior art fastener is used, the time spent installing and removing such fasteners can represent a significant fraction of production and labor costs. Due to the inherently inadequate clamping pressure provided by these traditional mechanical fasteners, skilled persons must install and remove such fasteners. Therefore, automating the installation and removal of mechanical fasteners appears unlikely given the current mechanical temporary fastener designs. Therefore, there is a need in the art to provide a fastener that can provide adequate clamping pressure and that can be installed and removed in an automated system, i.e. installation and removal by robot or other automated process.
The present invention addresses these and other needs as described below.